This application is a continuation of application Ser. No. PCT/98/04175, filed Jun. 17, 1998.
The present invention relates to a remote loop-back device intended to frequency-transpose a microwave signal of frequency f1 into a microwave signal of frequency f2. The application of the invention lies, more particularly, in the production of equipment for directional millimetre radio.
Radio links are organized around terminal stations and relay stations. Since radio beams are two-directional, each of these stations has a reception system and a transmission system. A terminal station 1 is represented in FIG. 1. This station is connected to an antenna 2 via a waveguide 3. As indicated above, it has a transmission system 4 and a reception system 5. The working frequencies of these systems are respectively the frequencies f1 and f2, of the order of thirty gigahertz. In order for the antenna 2 to be used simultaneously for transmission and reception, the station furthermore has a duplexer 6. Finally, in order to detect any malfunction in one of its elements, the station conventionally comprises a remote loop-back device 7 which, as it.were, is a circuit for monitoring the operation of the station. This device is interposed between the antenna 2 and the duplexer 6. During the station""s transmission/reception mode, this device is fully transparent and is not involved in the transmission of the signal of frequency f1 or in the reception of the signal frequency f2. However, when this device is activated, it makes it possible to loop the transmission system of the station back onto its reception system in order to ensure that the terminal station is operating correctly. Of course, when it is active, the terminal stations or the relay stations which are in communication with the terminal station in question are put on standby.
In order to perform this loop-back, the remote loop-back device carries out frequency transposition on the signal of frequency f1 output by the transmission system in order to convert it into a signal of frequency f2 which the reception system of the terminal station can use.
At present, the most commonly used remote loop-back device is produced using a diode and a local oscillator which resonates at the frequency f1-f2 (referred to as the duplex frequency fd). It is represented in FIG. 2. It comprises a diode 8 which is mounted inside the waveguide 3 joining the antenna 2 to the duplexer 6. The anode of the diode is connected to the wall of the waveguide, and its cathode is connected via a wire to the local oscillator. The portion of the wire arranged inside the waveguide makes it possible both to pick up the signal frequency f1 propagating in the waveguide 3, convey the signal of frequency f1-f2 to the diode which carries out the mixing, and transmit the signal of frequency f2 resulting from the mixing of the signals of frequency f1 and fd in the diode. Since the diode 8 is a non-linear component, it actually mixes the signal of frequency f1 with the signal of frequency f1-f2 output by the local oscillator, and consequently produces a signal of frequency f2 which the reception system of the station will be able to use. Furthermore, in order to be fully effective, the diode 8 is positioned in the middle of one side of the waveguide and its axis is parallel to the axis of the electric field E of the electromagnetic waves propagating in the waveguide.
Although very widely used, this device nevertheless has certain drawbacks. First, the presence of the diode inside the waveguide interferes with the propagation of the waves in the waveguide. It constitutes an obstacle to the transport of the wave in the guide, and this results in the appearance of a reflection coefficient at this point in the waveguide. Furthermore, all the parameters relating to the absorption of the frequency f1 by the device as well as those relating to the level of mixing in the diode are dependent on the position of the diode and the wire in the waveguide, and on their dimensions. It is consequently very difficult to set these absorption and mixing level parameters because this is generally done by xe2x80x9ctwistingxe2x80x9d the wire until the desired parameter values are obtained.
The object of the invention is to overcome these drawbacks by providing a remote loop-back device which allows simpler and faster adjustment.
The invention proposes to place the elements of the remote loop-back device outside the waveguide so as not to interfere with the propagation of the waves travelling in the waveguide. It also proposes to use lossless lines of defined length for transporting the microwave signals outside the waveguide.
The invention therefore relates to a remote loop-back device intended to frequency-transpose a microwave signal of frequency f1 propagating in a waveguide into a microwave signal of frequency f2, the transposition being carried out using a diode and a local oscillator which resonates at the frequency f1-f2, with f1-f2 less than  less than f1 or f2, the signals of frequency f1 and f2 both having wavelengths close to a value xcex,
characterized in that the device is mounted outside the waveguide, and in that it includes:
a first line of length xcex/2, a first end of which is slightly inserted into the waveguide and serves as an antenna for receiving the signal of frequency f1 as well as for transmitting the signal of frequency f2, and a second end of which is connected to a first electrode of the diode,
a second line of length xcex/2, the first end of which is connected to the first electrode of the said diode and the second end of which is unconnected in order to return a very high impedance to the first end of the first line;
a third line whose length is close to xcex/4 but without being equal to this value, the first end of the said third line being connected to a second electrode of the said diode and its second end being unconnected,
the fourth line of length xcex/4, the first end of which is connected to the second electrode of the said diode and the second end of which is connected, on the one hand, to a stub and, on the other hand, to the output of the local oscillator, the stub being intended to return a zero impedance for the signals of wavelength xcex on the second end of the fourth line, and
a fifth line of length xcex/4, the first end of which is connected to the first electrode of the said diode and the second end of which is earthed.
Advantageously, the remote loop-back device furthermore includes a first series of line extension elements beyond the unconnected end of the second line, which can be connected thereto by welding in order to adjust its length and thus to adjust the impedance returned to the unconnected end of the first line. The absorption level of the device is adjusted in this way.
The remote loop-back device furthermore includes a second series of line extension elements beyond the unconnected end of the third line, which can be connected thereto by welding in order to adjust its length and consequently to adjust the mixing level of the signals in the diode.